The present invention relates to an optical pick-up of an optical disc apparatus that is able to record/reproduce data of a plurality of kinds of optical discs whose cover layers have different thickness. Particularly, the present invention relates to the optical pick-up that has a combination of a refractive lens element and a diffractive element.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer. A laser beam from the optical pick-up forms a beam spot on the information layer through the cover layer. The distance between the optical pick-up and the information layer varies depending upon the thickness of the cover layer.
Namely, the thicker the cover layer is, the farther the distance to the beam spot from the optical pick-up is. For example, since the cover layer of a compact disc (CD) or that of a CD-R has the thickness of 1.2 mm, and the thickness of the cover layer of a digital versatile disc (DVD) is 0.6 mm, the optical pick-up is required to move the beam spot away from the optical pick-up by 0.6 mm in the cover layer (0.4 mm in air) when the DVD is replaced with the CD or the CD-R.
Although a paraxial beam spot moves as the objective lens is moved, the change of the thickness of the cover layer changes spherical aberration. If the optical pick-up only moves the objective lens when the disc is replaced, wave front aberration of the laser beam is deteriorated. Thus, the diameter of the beam spot is increased, which prevents the optical disc apparatus from reproducing the recorded information from the CD. For instance, when the objective lens, which is designed to minimize the spherical aberration when the recorded information is reproduced from the DVD, is used for reproducing the information from the CD, the spherical aberration becomes too large to reproducing the information even if the objective lens moves to bring the beam spot to be coincident with the information layer.
Therefore, there is known as prior art, an optical pick-up that adjusts the condition of the laser beam entering into the objective lens according to the thickness of the cover layer.
For example, Japanese Provisional Patent Publication No. HEI 7-98431 discloses such an optical pick-up. The optical system shown in this publication employs a holographic lens on the laser source side of the objective lens to divide the laser beam from the laser source into a zero order parallel diffractive beam and a first order divergent diffractive beam. The zero order diffractive beam is used for the optical disc having a thinner cover layer (i.e., the DVD)) the first order diffractive beam is used for the optical disc having a thicker cover layer (i.e., the CD and CD-R). The optical pick-up of the publication enables to form the diffraction-limited beam spots for the respective optical discs when the holographic lens is designed to obtain the most suitable laser beams according to the thickness of the cover layers.
However, since the optical pick-up of the publication always divides the laser beam from the laser source into the zero and first order diffractive beams, and only one of these beams is used for recording/reproducing information at a time, the maximum efficiency in use of the light quantity is not more than 40%.
Further, when one of the diffractive beams is being used for recording/reproducing the information, the other diffractive beam is an unnecessary beam, which increases noise.
Still further, the recording density of the DVD is higher than that of the CD, which requires the optical pick-up for the DVD to form a smaller beam spot than the optical pick-up designed for the exclusive use of the CD (hereinafter referred as an exclusive CD pick-up). Since the diameter of the beam spot has a positive correlation with the wavelength of the laser beam, the optical pick-up for the DVD requires the laser source whose oscillation wavelength is 635 through 660 nm that is shorter than the oscillation wavelength of the exclusive CD pick-up (i.e. 780 through 830 nm). On the other hand, the reflection characteristics of the CD-R requires the laser source whose oscillation wavelength is about 780 nm.
Accordingly, when the optical pick-up having a single laser source as described in the publication employs a laser source that emits a laser beam having a shorter oscillation wavelength, it cannot reproduce the information from the CD-R.
It is therefore an object of the present invention to provide an objective lens for an optical pick-up, which is capable of recording/reproducing information on a plurality of kinds of the optical discs (e.g., CD, CD-R and DVD) whose cover layers are different in the thickness. Further, the present invention is aimed to provide a composite objective lens that has higher efficiency in use of the light quantity than the conventional optical pick-up as disclosed in the above-identified publication.
For the above object, according to the present invention, there is provided an improved optical pick-up, which includes:
a plurality of light sources for emitting light beams having different wavelength, the light sources being switched with each other according to the kind of optical disc, used;
a refractive lens element for converging the light beams from the light sources onto a recording layer of the optical disc; and
a spherical aberration correcting element on which a concentric phase grating structure is formed, the phase grating structure altering spherical aberration in response to change of wavelength to correct change of the spherical aberration due to change of the thickness of the cover layer.
With this construction, since the spherical aberration correcting element changes spherical aberration corresponding to a change of the wavelength of the laser beam, the switch of the laser sources can compensate a change of spherical aberration due to the change of the cover layer""s thickness.
The spherical aberration correcting element preferably has no paraxial power, and diffracts the light beams in the same diffractive order, for example, in a first diffractive order.
Further, the light source having shorter emitting wavelength may be used for the optical disc having a thinner cover layer that has higher recording density, and the light source having a longer emitting wavelength may be used for the optical disc having a thicker cover layer that has lower recording density. In such a case, the peripheral area of the spherical aberration correcting element is preferably optimized for the optical disc having the thinner cover layer with the shorter wavelength. The peripheral area is the outside of an effective diameter corresponding to numerical aperture required for the optical disc having the thicker cover layer. In other definition, the peripheral area is the outside of a 85% line of an effective diameter of the spherical aberration correcting element. The peripheral area may be formed as a continuous surface or a grating surface. In the later case, a blazed wavelength of the peripheral area should be shorter than that of the central area.
The spherical aberration correcting element maybe located between the light sources and the refractive lens element. The refractive lens element and the spherical aberration correcting element constitute a composite objective lens. The composite objective lens may be designed for an infinite system in which parallel light beams are incident on the lens or a finite system in which divergent light beams are incident on the lens. In the infinite system, a collimator lens is required between the light sources and the composite objective lens.
Further, the spherical aberration correcting element preferably has a wavelength dependence such that spherical aberration varies in the undercorrected direction as wavelength of incident light increases.
As described above, the spherical aberration varies in the overcorrected direction as the thickness of the cover layer increases. Therefore, when a longer wavelength laser source is used for an optical disc having a thicker cover layer, and a shorter wavelength laser source is used for an optical disc having a thinner cover layer, the change of the spherical aberration due to change of the cover layer""s thickness is corrected by the above-mentioned wavelength dependence of the spherical aberration correcting element.
An additional optical path length added by the phase grating structure is expressed by the following optical path difference function "PHgr"(h):
"PHgr"(h)=(P2h2+P4h4+P6h6+ . . . )xc3x97xcex
where P2, P4 and P6 are diffractive coefficients of second, fourth and sixth orders, h is a height from the optical axis and xcex is wavelength of incident light.
The phase grating structure of the spherical aberration correcting element may satisfy the following condition (1);
xe2x88x9215 less than "PHgr"(h45)/xcexxe2x88x92P2xc3x97(h45)2 less than xe2x88x927xe2x80x83xe2x80x83(1)
where h45 is the height from the optical axis of a point where a light ray whose NA is 0.45 intersects the phase grating structure.
Preferably, one surface of the spherical aberration correcting element is a continuous surface and the other surface thereof is the grating surface. A base curve of the grating surface may be a flat plane or a rotationally symmetrical aspherical surface. The base curve is defined as a shape of the surface that does not include the phase grating structure.